1. Field of the Invention
This invention relates to a charging member and a charging device which can be utilized in an image forming process such as an electrophotographing process or electrostatic recording process.
2. Description of the Related Art
In the charging process of electrophotography high voltage (DC 5 to 8 kV) is conventionally applied between a shielding plate and a metal wire partially surrounded by the plate to generate corona, by which a photosensitive member is charged. In this process, however, corona products such as ozone and NOx produced by the corona generation cause several problems such as deterioration of a photosensitive member surface, blurring or degradation of an image. Further, wire contamination caused by the products affects the quality of an image, resulting in white stripes or black stripes in an output image. From an electrical aspect also, the process is not effective, because most of loaded electrical current flows into the shielding plate surrounding the wire, and it is only 5 to 30% of the current that flows into the photosensitive member.
In order to overcome these problems, direct charging devices in which a charging member is contacted with a member to be charged such as a photosensitive member has been proposed, as is shown in the Japanese Unexamined Patent Publications JPA 57-178267 and JPA 56-104351. Several types of charging members are conventionally employed for directly charging. These are (a) a conductive rubber roller with a metal core having been coated with a rubber material such as butadiene-acrylonitrile rubber (NBR), acrylic rubber, or chloroprene rubber, whose resistivity is adjusted by dispersing low-resistivity particles such as carbon black, and (b) a roller coated with nylon or polyurethane as disclosed in the Japanese Examined Patent Publication JPB2 50-13661.
However, the conductive rubber roller (a) has disadvantages in that the hardness of the rubber is relatively high because a relatively large amount of the low resistivity particles is dispersed in the rubber in order to maintain its low resistivity, and that the surface of the member to be charged such as a photosensitive member can be damaged due to the low resistivity particles present in the surface of the rubber. Such damages likely lead to image defects such as stripes. Although the addition of oil or the like may lower the hardness of the roller, it brings about another problem that the oil excludes from the rubber roller surface and then contaminates the surface of the photosensitive member.
In the case of the roller coated with nylon or polyurethane (b), the charging capability greatly varies depending on environment, particularly, humidity. For example, at a low temperature and a low humidity, the charging capability lowers extremely. With such decreased charging capability, uniform charging of the member to be charged cannot be realized. This results in the decrease of image density in image formation. Black spots due to nonuniform charging arise in a negative development system, while white spots arise in a positive development system. In any case, high quality images can not be produced and, therefore, the image-formation is prone to environment.